The Dependence of Radius on Relative Humidity and Solute Mass at High Relative Humidities Up to and Including 100%

Abstract

This manuscript presents a rigorous examination of the equilibrium radius ratio of hygroscopic aerosol particles that are solution drops of ideal and actual inorganic solutes for a range of solute masses in the relative humidity range 99–100%. Analysis is presented first for ideal solutes over a range of solute masses for different hygroscopicities, providing a theoretical framework for the subsequent analysis of actual solutes. Dimensionless quantities are used to reduce the Köhler equation to one with no free parameters and an approximate solution is presented. Next analysis is presented for three common inorganic solutes of atmospheric importance: sodium chloride, ammonium sulfate, and sulfuric acid, with a range of solute masses considered for each substance. The extent to which nonideality is important for these solutes over the ranges of relative humidities and solute masses considered is examined. A simple but accurate single‐parameter expression is presented for the equilibrium radius ratio as a function of relative humidity that contains the dependences on solute mass and composition for these solutes. The accuracy of this expression is quantified and factors that limit this accuracy are discussed. The expression is accurate to within 5% over most of the range of mass‐equivalent dry radii from 10 to 250 nm and relative humidities from below 99 up to 100%, over which the equilibrium radius ratio varies by more than a factor of 5, and it remains finite at 100% relative humidity. Finally, measurements of hygroscopic growth of aerosol particles in this relative humidity range are discussed.

Concentrations and Adsorption Isotherms for Amphiphilic Surfactants in PM1 Aerosols from Different Regions of Europe

“…They show that σ for PM 1 particles is lower than σ water in 98% of the samples for GF = 5, and in 60% of them for GF = 10. As, for submicrometer particles containing a significant organic fraction, GF at activation is estimated to be between 2 and 10, 32 this suggests that the σ of submicrometer particles was lower than σ water at activation in at least 60% of the samples at these sites. Note, however, that the above calculations provide only lower limits for GF, as they are based on the concentrations measured in this work, thus averaged over the entire PM 1 volume for each sample.…”

Effects of Salinity Beyond Coalescence on Submicron Aerosol Distributions

“…(2017), which is lower than reported values of NaCl, likely due to hydrates found in the sea salt (Tang et al., 1997; Zieger et al., 2017). We assume that the κ value is constant across all concentrations, however it is known that κ is a function of water activity, and consequently the relative humidity and D dry (Lewis, 2019). The Kohler theory itself also has limitations, often due to the practical osmotic coefficient varying from unity (Lewis, 2008).…”